1. Technical Field
The present invention is directed to an actuatable occupant restraint system for a vehicle and is particularly directed to an apparatus for testing the operativeness of a squib of such a restraint system.
2. Description of the Prior Art
Air bag restraint systems for passenger vehicles are known in the art. Such systems typically include an actuation circuit and a diagnostic circuit. The actuation circuit has an associated, electrically actuatable igniting device, referred to as a squib. The system further includes a crash sensing device, such as an inertia sensor, for sensing the crash acceleration of the vehicle. When the inertia sensor is subjected to a crash acceleration greater than a predetermined value, the inertia sensor closes an electrical switch.
The electrical switch and the squib are connected in series across a source of electrical energy. When the electrical switch is closed as a result of vehicle crash acceleration greater than a predetermined level, electric current of sufficient magnitude and duration ignites the squib. The squib, when ignited, ignites a combustible gas generating composition and/or pierces a container of pressurized gas operatively connected to the air bag, which results in inflation of the air bag. Squib ignition can also control other actuatable restraints such as seat belt pretensioners, seat belt locks, knee bolsters, etc.
If the actuation circuit is inoperative, the restraint would not be actuated upon the occurrence of a crash condition. Such inoperative conditions include (i) an open circuited squib, (ii) a short circuited squib, or (iii) a short between a squib terminal and the supply voltage, and (iv) a short between a squib terminal and electrical ground.
The diagnostic circuit tests the operativeness of the actuation circuit and controls an indicator to inform the vehicle operator upon the occurrence of a detected system error. Known restraint system diagnostic circuits typically include numerous switching components to monitor voltage values at various test points in the firing circuit. Prior art diagnostic systems use various methods or algorithms for calculating firing circuit component operating/performance parameters using the monitored voltage values. These calculated component values are compared against associated limits. When an open or short circuit condition exists or a calculated component value is outside of its associated limits, an error or fault condition has occurred and an indicator is energized.